Howdy all!

I have a few questions for the learned people here, please forgive me if they seem basic and n00bish, I've just started taking electrical courses in school and we haven't got to some of the fancy stuff yet.

OK, here goes!

I don't understand how the transistor (2STX2220) in the Peggy 2LE matrix can deal with turning on 1 LED or 25 LED's. Can it sense how much voltage or current is needed depending on how many LED's are being lit? Or is it being controlled by the input signal on the Base and outputting the voltage and current needed?

Vcc is 5V and 1.2A right? 1.2A will fry most LED's but I don't see and current limiting resistors between Vcc and the LED's so the transistor deals with this yes? Does the transistor also amplify the 5V to light the entire row of LED's if necessary, or are each LED switched on/off individually very fast?

Thanks again for any input! I've googled for days but it's hard to understand the applications. I'm thinking about trying to build myself an LED matrix, but we don't get to transistors etc for another semester or so

Thanks!
Mark

I don't understand how the transistor (2STX2220) in the Peggy 2LE matrix can deal with turning on 1 LED or 25 LED's. Can it sense how much voltage or current is needed depending on how many LED's are being lit?

The transistor basically connects the high side of all 25 LEDs to Vcc (5 V). Then the "column driver" chips -- the "LED drivers" --connect the low side of selected columns to ground, each through a current-limiting circuit that is inside the chip. LEDs that have both (1) their high side connected to Vcc and (2) their low side connected to ground will turn on. LEDs that have only their high side connected, or only their low side connected, do not turn on.

Vcc is 5V and 1.2A right?

No. Vcc is 5 V. It's a voltage, not a current. I'm not sure where you're getting the 1.2 A figure from.

don't see and current limiting resistors between Vcc and the LED's so the transistor deals with this yes?

No. The transistor basically is an on-off switch in this context, simply connecting all the LEDs in that row to 5 V. The current is *regulated* by the LED driver chips, which are set to allow 20 mA, no more.

Aaaahhhhhh, makes more sense now, I had been too focused on the matrix portion and forgot to look at the rest of the schematic I feel a little silly, completely overlooked the LED driver chips

No. Vcc is 5 V. It's a voltage, not a current. I'm not sure where you're getting the 1.2 A figure from.

I thought I had seen it on the schematics somewhere, but now looking over it again, I can't find it. Figment of my imagination I guess

Thanks for your info Windell
Mark